1. Field of the Invention
This invention relates in general to automatic control of torque, force, or tension acting on a stationary or moving object, called the payload, when the torque or force is produced by a motor with a stationary frame and a movable element that is connected to the payload. The invention relates in particular to systems for which there is no measurement or sensing of torque or force at the payload.
2. Prior Art
Control is typically achieved by means of a negative feedback control system or servomechanism. A typical servo is comprised of a motor that drives the load and an amplifier that supplies power to the motor. The motor output torque or force is sensed by a transducer which feeds a proportional signal back to a summing junction where it is subtracted from a signal that is proportional to the command torque or force. The difference in signals produces an error voltage which is fed into the amplifier which then sends power proportional to the error to the motor. The motor then drives the load until the error is reduced to an acceptably small value or is eliminated. It is noted that the terms motor and amplifier are used in a general sense. The motor may be electric, hydraulic, or pneumatic and the power amplifier must supply the type of power that is appropriate to the type of motor. Sometimes a speed convertor or gear train is connected between the motor and load.
Probably the most common and complex application is tension control of a flexible membrane that is connected to and wound around a drum or reel. Examples of applicable membranes are thin flat sheets such as paper, fabrics, plastic film, and metal foil and circular strands, such as thread, wire, cable, and rope. This application is used to describe the chief embodiment of the invention.
The reel must follow the motion of the membrane in order to maintain the desired membrane tension. When the membrane is moving toward the reel during winding, if the reel moves too slowly the membrane may become slack and if the reel moves too fast the membrane may break. When the membrane is moving away from the reel during unwinding, just the opposite may occur when reel speed is too slow or too fast. Since the membrane is elastic, some tension error can usually be tolerated before slack forms or rupture occurs. Accurate tension control is often important for other reasons, such as quality control of processes performed during the manufacture of the membrane.
In some applications, the membrane is also attached to and wound around another reel, located some distance from the tension control reel. In this system, the second reel controls motion, usually by means of velocity or position servos, and is called the master reel. The tension control reel is called the slave reel, since it must follow the motion dictated by the master reel. Unless otherwise specified, the term "reel" means the slave reel or tension control reel.
There are devices which can sense membrane tension directly, but they are awkward, expensive, and may damage the membrane when they require physical contact with the membrane. Some sensors require that the membrane travel through sets of rollers that subject the membrane to small bend radii, high bending stress, and abrasion. This problem increases for membranes with low flexibility, material strength, and hardness. Another problem is unpredictable variations in membrane characteristics, such as spring rate, which can cause errors in tension measurement. Such variation is likely to occur during manufacture of the membrane.
An indirect method of controlling tension is to control motor output torque in conjunction with the proper torque commands. One method of producing the proper torque command is to sense reel position, which is used to compute the outer radius of the reel. Command torque is computed as the desired tension multiplied by the radius. This method doesn't require any sensor contact with the membrane and is used in the invention.
Controlled torque is feedback torque which is torque sensed at the motor. This method often results in intolerably large transient errors, however, unless changes in speed and/or position occur very slowly.
One type of transient error is proportional to the product of inertia and acceleration; the desired force at the payload, or membrane, is less than controlled force during acceleration and greater during deceleration (negative acceleration). During acceleration, the force available for the payload is the controlled torque minus the torque required to accelerate the inertia of the motor and reel, divided by radius.
A second type of transient error is proportional to the product of external damping and velocity. External damping might result from viscous damping or windage in an electric motor or a torsional damper connected to the motor or reel to cause rapid decay of oscillations. The desired torque at the payload is reduced by the above velocity error. (The effects of internal damping, such as results from back emf in an electric drive or fluid flow losses in a hydraulic drive, are located within the servo loop and are thus minimized or elliminated by negative feedback.)
The motor output torque at zero or fixed speed may be measured by a load cell fastened between a torque arm and the machine mounting frame. This torque is the product of force measured by the load cell and torque arm radius. During acceleration, motor output torque is the above measured torque minus the product of the inertia of the moving element of the motor and its acceleration. The measured torque will also be called steady state motor output torque, herein. This method is generally used when a rotary hydraulic motor is used.
A less accurate method somtimes used for hydraulic or pnuematic motors, is to measure and control the pressure drop between the supply and return ports of the motor. This pressure differential does not account for the pressure and torque losses due to flow resistance and mechanical friction in the motor, which are significant in hydraulic motors.
The technique generally used for electric motors is to sense motor current, which is proportional to developed motor torque at fixed temperature. The torque/current ratio typically drops by 5% as the motor heats up to its allowable operating temperature. The steady state motor output torque is slightly less than developed torque due to motor friction and internal viscous damping losses, such as windage, but these are generally negligible. During acceleration, motor output torque decreases as stated above.